1. Field of the Invention
This invention relates to speedometer testers and calibration devices, and in particular to a mobile speedometer tester.
2. Background of the Invention
Speedometers are speed indicators which display the rate at which a vehicle is traveling. These devices are standard equipment on land vehicles such as cars, tracks and motorcycles. Speedometers are important in that they allow vehicle operators to control their speed fairly accurately for purposes of fuel economy, compliance with the applicable speed limit, etc. Officially calibrated speedometers installed on official vehicles such as police cars may be used to check the speed of other motorists, and used as a basis for issuing speeding citations. For that reason, police speedometers must be regularly tested. In addition, speedometers mounted on non-official vehicles may periodically require testing.
3. Existing Designs
Over the years, a number of approaches have been taken toward developing speedometer testers. U.S. Pat. Nos. 2,607,212, 2,781,659 and 3,178,927 were granted for in-floor speedometer testers. These designs comprised two pairs of parallel rollers embedded in the floor of the testing facility. The drive wheels of the vehicle to be tested were positioned between the pairs of rollers so as to be supported by the rollers. Any rotational movement of the drive wheels would impart a corresponding rotational movement to the drive wheel support rollers. The vehicle's drive wheels were then accelerated to the different speeds at which the speedometer was to be checked, and the roller speed measurement where the roller diameter was accurately known, the drive wheel rim speed could be tellably measured, and compared against the vehicle speedometer reading.
Problems associated with this in-floor type of speedometer tester design included heavy and cumbersome equipment, expensive, permanent installations, and lack of transportability of the testing apparatus. This last disadvantage was especially onerous: all vehicles to be tested had to be driven to the testing facility to have their speedometers checked. Where an entire police department's vehicles had to be tested, this procedure resulted in considerable wastage of man hours driving to and from the testing facility. This could be expensive where the police officers had to do the driving. Finally, no provisions were made to test motorcycle speedometers. This is a non-trivial problem, because motorcycle speedometers are typically mounted on a different wheel from the drive wheel--generally a motorcycle's rear wheel is the drive wheel, while the motorcycle speedometer sensor is connected to the motorcycle front wheel. Therefore, an independent means to impart rotational motion to the wheel connected to the motorcycle speedometer is required.
Attempts to solve the transportability problem have met with varying degrees of success. U.S. Pat. Nos. 3,435,657 and 4,185,489 were granted for transportable speedometer testers which could be attached to a vehicle drive wheel. Tho drive wheel was then jacked off the floor and accelerated to the different speeds at which the speedometer was to be checked, and the rotational speed noted. A major problem with this procedure was that the testing apparatus yielded the drive wheel rate of rotation, not the actual drive wheel rim speed. Any rim speed calculation based on the testing apparatus' rotation rate reading was therefore necessarily approximate, because the circumference of the drive wheel could only be approximated. Even small mounts of drive wheel wear would throw the results off, became tire wear reduces drive wheel circumference. Even more importantly, the circumference of a non-loadbearing wheel is greater than the circumference of a wheel beating a quarter of the weight of a car (around 800-1000 lbs.), became the air within a loadbearing tire will compress, reducing wheel circumference.
A third type of speedometer tester required the disconnection of the vehicle speedometer drive cable from the speedometer to be tested, and the attachment of the tester directly to the speedometer being tested. Patents representative of this variety of speedometer include U.S. Pat. Nos. 3,602,034, 4,070,901, 5,027,640 and 5,040,407. Some inventions even required the additional intermediate step of connecting a ratiometer to the vehicle speedometer and driving the vehicle a set distance, in order to calibrate the testing apparatus
Disconnection of the test vehicle speedometer involved getting behind the test vehicle dashboard. In some cases the test vehicle dashboard had to actually be removed--a laborious process. Disadvantages associated with this type of speedometer testing include increased complexity and time, and consequently higher cost In addition, whenever a procedure as complex as disassembling a dashboard, breaking the seal on a speedometer drive cable, and disconnecting the speedometer is undertaken, a certain amount of breakage risk is necessarily assumed. This increased risk further augments the cost of such testing. Finally, different shares have different laws regarding the circumstances under which a speedometer seal may be broken, which agencies are authorized to so do, and what procedures must be followed to reseal the speedometer. These regulations further inflate the cost of any speedometer testing procedure which calls for the speedometer in question to be disconnected.